1. SINUMERIK System 3 TT Control Architecture Overview
The SINUMERIK System 3 TT is a Siemens CNC platform built around the SIMATIC S5-90U / S5-95U family of compact PLCs for the automation/auxiliary axis logic layer, paired with the SINUMERIK numeric kernel that drives the turning (TT) machine's interpolators, spindle, and tool management. The TT variant is the turning-machine configuration of the System 3 family; the documentation is partitioned in Siemens' support library as "SINUMERIK System 3; Part 1 Basic Systems" with the TT entry listed as a related variant. Operators and service engineers will find the basic-system manual at the Siemens Industry Online Support portal under document ID 22090584.
From a service perspective, the SINUMERIK System 3 TT is a hybrid control:
- The NC kernel (path interpolation, servo loops, M/H function decoding) lives on SINUMERIK-specific submodules, typically EPROM, and is treated as read-only firmware.
- The PLC subsystem — the ladder/CSF logic that sequences tool changer, chuck, coolant, tailstock, door interlocks, and axis enables — is a SIMATIC S5 CPU with its own removable submodules. The CPU referenced on the System 3 TT control panel in field practice is the 6ES5 921-3W (S5-90U series compact CPU).
When service engineers need to modify the PLC program — adding a tool, changing an interlock, retrofitting a probe, or upgrading sequence logic — the EPROM submodules that hold the program must be replaced with writable RAM submodules. This is the standard workflow on every S5-based System 3 control, and it is the subject of this reference.
2. CPU 6ES5921-3W Identification and Role
The CPU 6ES5 921-3W belongs to the SIMATIC S5-90U compact controller family (6ES5 921 = S5-90U; 6ES5 922 = S5-95U, which adds integrated I/O and a second serial port). On the SINUMERIK System 3 TT, the 6ES5 921-3W acts as the auxiliary/sequence PLC — sometimes called the "machine PLC" — and communicates with the NC channel through a backplane interface.
| Property | Value / Description |
|---|---|
| Order number (MLFB) | 6ES5 921-3W |
| Family | SIMATIC S5-90U (compact PLC) |
| Programming languages | STEP 5 — LAD (ladder), CSF (function chart), STL (statement list) |
| Onboard RAM | Approx. 2 KB of integrated work memory for incremental program changes |
| Submodule slots | Two slots for memory submodules (6ES5 340 family) |
| Submodule roles | Slot 1 (upper): basic program / system blocks; Slot 2 (lower): PLC user program |
| Programming interface | PG via serial (TTY/PG 511 protocol) — requires CP 511 / AS 511 link |
3. Memory Submodule Topology — Two Slots, Two Roles
The 6ES5 921-3W accepts two 6ES5 340-series memory submodules. Each slot has a defined role, and swapping modules between slots is not a valid operation:
| Slot | Position on CPU | Typical Content | Type in Stock Machines |
|---|---|---|---|
| Slot 1 (upper) | First submodule socket | Basic program (system blocks OB, FB, PB required for PLC run-up and NC coupling) | EPROM (read-only; rarely modified in the field) |
| Slot 2 (lower) | Second submodule socket | PLC user program (application-specific blocks, sequencer, I/O maps) | EPROM on production machines; must be RAM for in-field editing |
The system is designed so that the basic program slot remains EPROM (or the matching Siemens-supplied system submodule), and the user-program slot is the one that gets replaced with RAM for service work. Both modules are physically keyed; module insertion direction is marked on the socket and the CPU front panel.
4. EPROM vs RAM: Why Service Work Requires a RAM Swap
EPROMS in the S5-90U family are one-time-writable (UV-erasable) memory submodules. The STEP 5 programmer cannot overwrite them in-circuit. To change logic, the engineer either:
- Replaces the EPROM with a RAM submodule, programs the new logic into RAM, and (after sign-off) burns a fresh EPROM; or
- Edits in onboard RAM — the S5-90U exposes roughly 2 KB of integrated RAM that STEP 5 can patch, holding the delta until the next submodule program/blow cycle.
Approach (1) is the clean workflow for any non-trivial change. Approach (2) — incremental edits in the 2 KB onboard RAM — works for small FB/PB corrections when a CP 511 / PG 511 online connection is available, but it does not replace the need for a RAM submodule when the change set is large or permanent.
5. Selecting a Compatible RAM Submodule (6ES5 340 Family)
The memory submodules for the S5-90U/S5-95U belong to the 6ES5 340 family. Service teams routinely use the part number 6ES5 340-3KB31 (a 16 KB RAM submodule in the same mechanical form factor as the EPROM submodules shipped on System 3 TT controls). Compatibility points to verify before insertion:
- Form factor / pinout: 6ES5 340 submodules use the standard S5-90U 40-pin submodule edge connector. Mismatched pinouts (e.g., 6ES5 341 for S5-100U) must not be forced.
- Capacity: Do not exceed the CPU's addressing window. For the S5-90U (6ES5 921), 16 KB submodules (3KB-series) are typical and supported.
- Battery backup: A 6ES5 340 RAM submodule must be backed by the CPU's backup battery (typically a 3.6 V lithium cell on the S5-90U). With a missing or depleted battery, the RAM contents are lost on power-down and the CPU will not start cleanly.
- Insertion direction: The submodule's component side faces the CPU; the keyed notch matches the socket. Reverse insertion is mechanically blocked by the key, but misaligned forcing will bend pins.
| MLFB | Type | Typical Use on System 3 TT |
|---|---|---|
| 6ES5 340-3KB31 | 16 KB RAM, battery-backed | Slot 2 (PLC user program) — service modification |
| 6ES5 340-3KBxx (EPROM family) | 16 KB EPROM, UV-erasable | Slot 1 (basic program); Slot 2 in production |
6. Required Programming Hardware — PG 511 and AS 511 / CP 511
Online programming of the S5-90U requires a Siemens programming device with the AS 511 protocol, typically delivered through the CP 511 communication processor card installed in the PG, or an equivalent PG with built-in AS 511. The "511 comms card" referenced in field discussions is the CP 511 / AS 511 interface — a serial current-loop (TTY) link between the PG and the S5 CPU's PG port.
| Item | Function | Notes |
|---|---|---|
| Siemens PG (e.g., PG 710, PG 730, PG 750) | Runs STEP 5 | STEP 5 V6.x or later; supports LAD/CSF/STL |
| CP 511 / AS 511 interface | Serial TTY comms to S5 CPU | Connects to the CPU's "PG" port (15-pin Sub-D on the S5-90U front) |
| STEP 5 project archive (on EPROM image) | Source for the re-program | Read existing EPROM first; preserve the program on a separate EPROM before overwriting |
| UV EPROM eraser (optional) | For re-blowing fresh EPROMS at sign-off | Only required when burning a new permanent EPROM image |
Without the CP 511 / AS 511 link, the PG cannot reach the S5-90U's work memory, and the only way to change the program is physical EPROM swap with an external EPROM programmer — workable but slow.
7. EPROM-to-RAM Conversion Procedure
The procedure below converts the System 3 TT's PLC program from EPROM to a writable RAM submodule on the 6ES5 921-3W CPU, while leaving the basic-program EPROM in slot 1 untouched.
- Document the existing configuration. Photograph the CPU faceplate; record both submodule MLFBs, firmware version (if printed), and the project name visible in the PG. Confirm the control is in a safe state — main contactor off, drives de-energized, and any stored energy discharged per lockout/tagout (LOTO).
- Back up the existing EPROM contents. With the PG connected online via CP 511, perform an EPROM → PG upload of the program. Save the binary (and ideally the symbol/comment file) to PG storage or a separate EPROM. This is your recovery point if the modification fails.
- Power down the control. Switch off the SINUMERIK System 3 TT per its standard power-down sequence, and confirm the backup battery is healthy (≥ 3.0 V on the S5-90U lithium cell) before removing power. The RAM submodule will rely on this battery.
- Remove the lower EPROM submodule (slot 2). Note the orientation. Place the EPROM in a conductive foam or shielded container — EPROMS are vulnerable to static discharge and UV.
- Insert the 6ES5 340-3KB31 RAM submodule into slot 2. Verify keyed alignment, seat firmly but without force, and confirm the component side faces the CPU.
- Reapply power and observe CPU run-up. The PLC should reach the RUN state. If the CPU reports a submodule fault, power down, reseat the RAM, and check the battery.
- Download the saved program into the new RAM submodule. From STEP 5, perform a PG → RAM transfer of the previously uploaded binary. Verify block-by-block: OB, FB, PB, SB, DB.
- Make the required logic changes in STEP 5. Use the integrated 2 KB onboard RAM for small in-test edits, or operate directly in the RAM submodule for larger modifications.
- Test the modified logic in a controlled mode. Use the S5-90U's test functions (status, single-step) and the System 3 TT's NC-side dry-run mode where available. Validate every modified PB on the machine before production reset.
8. Burning a Fresh EPROM (Permanent Image)
Once the modified logic is signed off in RAM, the change should be frozen onto a UV-EPROM for production deployment. Siemens part numbers in the 6ES5 340 EPROM family accept standard EPROM programmers (Data I/O, BP Microsystems, etc.) using the Siemens S5 memory image format. Steps:
- Export the current RAM image from STEP 5 to the PG.
- Write the image to a blank UV-EPROM using the EPROM programmer; verify the blank check, write, and verify cycles.
- Power down the System 3 TT, swap the RAM submodule for the new EPROM in slot 2, and reapply power.
- Confirm the CPU reaches RUN with the EPROM image, then archive both the new EPROM and the source binary.
9. Verification and Commissioning Checks
After the RAM submodule is in place and the new program is loaded, the following checks confirm a clean conversion:
| Check | Method | Pass Criterion |
|---|---|---|
| CPU run-up | Observe S5-90U front LEDs after power-on | RUN LED steady, no submodule/battery faults |
| Submodule contents | STEP 5 PLC → Submodule Info | Slot 2 reports RAM type with expected block list |
| Battery health | STEP 5 status page or multimeter on test pins | ≥ 3.0 V lithium; no BATTF indicator |
| NC ↔ PLC handshake | Watch SINUMERIK System 3 TT status page for PLC ready | PLC READY / NC READY interlock active |
| I/O mapping | Force-test inputs and outputs in STEP 5 status | All digital I/O points respond per the wiring diagram |
| Sequence dry run | Execute tool change / chuck cycle in dry-run | All sequencer transitions follow modified PB logic |
10. Troubleshooting Matrix
| Symptom | Likely Cause | Action |
|---|---|---|
| CPU does not enter RUN after RAM insertion | RAM submodule not fully seated, wrong type, or battery dead | Power down; reseat; check MLFB (6ES5 340 family); test/refresh backup battery |
| Submodule fault LED on at power-up | Inserted into wrong slot, or EPROM/RAM mismatch with system expectations | Verify slot 1 keeps the basic-program EPROM; slot 2 holds the RAM |
| PG cannot connect via AS 511 | CP 511 cable/TTY loop fault, wrong port, or PG baud/parity | Loop back test; replace TTY cable; verify AS 511 protocol settings in STEP 5 |
| Edited block reverts on power cycle | Backup battery depleted; edits were made only to onboard 2 KB RAM | Replace lithium cell; promote changes into the RAM submodule so they survive power-down |
| Sequence logic differs from program listing | Compiled/cross-reference mismatch after PG → RAM transfer | Re-upload the program; verify block consistency (no MIX markers) and re-cross-reference |
11. Safety and Handling Notes
- Always discharge stored energy on the machine's DC bus and 24 V supplies before opening the control cabinet. The SINUMERIK System 3 TT is a high-energy CNC; partial power-down is not a service condition.
- EPROMS and RAM submodules are ESD-sensitive. Use a grounded wrist strap and a conductive mat when handling.
- Do not expose windowed EPROMS to direct sunlight or fluorescent UV; doing so will erase the program.
- Label every removed submodule with the machine serial, slot position, and the date. A confused submodule returns to the wrong slot only on machines where it has happened before.
- Maintain at least one known-good EPROM copy of the running PLC program off-machine, both as a spare part and as a disaster-recovery image.
12. Frequently Asked Questions
Which memory submodule slot holds the PLC user program on a SINUMERIK System 3 TT with CPU 6ES5921-3W?
On the 6ES5 921-3W, the lower (second) submodule slot holds the PLC user program; the upper slot holds the basic program. Only the lower slot should be converted from EPROM to RAM for in-field editing.
Can a 6ES5 340-3KB31 RAM module be used as a direct replacement for the user-program EPROM?
Yes — the 6ES5 340-3KB31 (16 KB battery-backed RAM) is mechanically and electrically compatible with the S5-90U submodule socket and is the standard service-replacement for the user-program EPROM. Confirm the exact MLFB against the SINUMERIK System 3 Part 1 manual (Siemens document 22090584) before ordering.
What is required to program the S5-90U online from a PG?
A Siemens PG running STEP 5 with a CP 511 (AS 511) communication card and a TTY cable connected to the CPU's PG port. Without an AS 511 / CP 511 link, the only way to change the program is to physically remove and reprogram the EPROM with an external device.
Can the original EPROM contents be edited directly without changing to RAM?
No. EPROMS on the S5-90U cannot be overwritten in-circuit. For non-trivial logic changes, replace the user-program EPROM with a RAM submodule, modify in RAM, then re-burn a fresh EPROM once the changes are signed off.
Will the modified program survive a power-down after converting to RAM?
Yes, provided the S5-90U's backup lithium battery is healthy (≥ 3.0 V). A depleted battery will cause the RAM to lose contents on power-down, leaving the CPU unable to start from the user-program slot. Test/refresh the battery at every service intervention.